Electrorheological (ER) fluids display remarkable rheological behavior, being able to convert rapidly and repeatedly from a fluid to a solid-like when an external electric field (E) is applied or removed. In this study, electrical and ER properties of poly(Li-2-hydroxyethyl methacrylate)-co-poly(4-vinyl pyridine), poly(Li-HEMA)-co-poly(4-VP), copolymeric salts (ionomers) were investigated. For this purpose six ionomers were synthesized with different molar masses. They were then ground-milled for a few hours to obtain micron size ionomers. The particle sizes of the ionomers were determined by dynamic light scattering. Suspensions of ionomers were prepared in silicone oil (SO), at a series of concentrations (c = 5-30%, m/m). The gravitational stability of suspensions against sedimentation was determined at constant temperature (T = 25 degrees C). Flow times of the suspensions were measured under no electric field (E = 0 kV/mm), and under an external applied electric field (E 0 0 kV/mm) strengths and a strong ER activities were observed for all the poly(Li-HEMA)-co-poly(4-VP)/SO suspensions. Further, the effects of suspension concentration, mole ratios of poly(HEMA) and poly(4-VP), and the overall molar mass of the copolymers, shear rate, electric field strength, frequency, promoter, and temperature onto ER activities of ionomer suspensions were investigated. (c) 2006 Wiley Periodicals, Inc.